string manipulation - Naming a pattern changes outcome in StringCases[]

s = "1 2 ";
StringCases[s, (n : NumberString ~~ " ") .. ]
StringCases[s, (NumberString ~~ " ") .. ]

yields

{"1 ", "2 "}
{"1 2 "}

Why?

Answer

Patterns get confusing quickly. If you name a pattern you're imposing more restrictions on that pattern that are sometimes difficult to follow. Using your example,

s = "1 2 ";
StringCases[s, (n : NumberString ~~ " ") .. ]

StringCases[s, (NumberString ~~ " ") .. ]

In the first case you're telling string cases to match n, where n must be a number string and to continue the pattern for any match with the parenthetical statement repeated. In the repeated suffix n must always be the same number!

In the second case you're specifying that any repeated pattern of NumberString+Whitespace should match. Since you haven't named the number string, the pattern still applies generally to any number.

Trying

 s = "1 1 1 2 2";
 StringCases[s, (n : NumberString ~~ " ") .. ]
 StringCases[s, (NumberString ~~ " ") .. ]

Will give:

{1 1 1 , 2 2 }
{1 1 1 2 2 }

Which shows that the first pattern works any time the integer following the white space is the same as the n that triggered the match.

Comments

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